STAR Flow, Chirality and Vorticity PWG

[Sooraj Radhakrishnan <skradhakrishnan AT lbl.gov>]

Hi Fuqiang,

   Thanks for the reference to the AMPT studies. I see the extraction of the non-flow estimate from AMPT on the delta gamma/v2 ratio, but I could not find a comparison between non flow estimates from HIJING and AMPT. In the paper, what I understand is that was not the focus and AMPT and HIJING are used for extracting different contributions. 

But I see from one of the references cited (https://arxiv.org/pdf/1912.00299.pdf), that in Au+Au, HIJING significantly over-estimates delta gamma/v2 than data and AMPT under-estimates (Fig.5). And in the discussions, one of the potential reasons mentioned is rescatterings resulting in weaker correlations in AMPT. But I guess this is not a fair comparison as v2 in HIJING means very different than in data or AMPT. I was thinking if a more direct comparison of the non-flow contribution between AMPT and HIJING could be made, like from the delta eta - delta phi correlations 

Yes, it is not very useful to have a discussion on which model implements correct non-flow. But my point was that since HIJING is missing a crucial part of heavy-ion collision evolution - one that can potentially alter the initial pair correlations - it would be good to check its impact. If the medium evolution has a significant impact on non-flow correlations, that would also change the conclusions on the impact of non-flow. If not, its a significant non-flow correction or estimate that we have to include in all flow measurements?

thanks,

Sooraj

On Thu, Jan 25, 2024 at 10:50 AM Wang, Fuqiang via Star-fcv-l <star-fcv-l AT lists.bnl.gov> wrote:

Hi Sooraj,

I’m not sure hijing is an overestimate of nonflow. I tend to think the opposite. 

AMPT has larger nonflow. I think AMPT is an overestimate of nonflow. We had some AMPT results on this in Phys. Rev. C 105, 024913 (2022), arXiv:2106.15595 [nucl-ex]. I think Jamie Nagle had an earlier publication reaching the same conclusion (I’ll have to dig out the reference).

In any case, we are not debating whether hijing or AMPT or other models can quantitatively describe nonflow in real data or not. The important point is that there’s nonflow contamination and the current full event and subevent estimate of 1-2% syst uncertainty is far from sufficient. One cannot simply ignore it and yet extract physics quantity based on data-hydro comparison. A more earnest assessment is needed. 

Best regards,

Fuqiang

On Jan 25, 2024, at 11:08 AM, Sooraj Radhakrishnan via Star-fcv-l <star-fcv-l AT lists.bnl.gov> wrote:



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Hi All,

   Sorry I could not join the meeting yesterday during the discussions. So if this was discussed, apologies 

HIJING could be overestimating non-flow as it does not have medium evolution/thermalization. For non-flow, the medium evolution would be contributing a decorrelating and diluting effect. 

Since AMPT uses HIJING as input, it would be good to see comparison of AMPT and HIJING. Looking at two particle delta eta - delta phi correlations should tell what the impact is 

Best,

Sooraj

On Thu, Jan 25, 2024 at 5:33 AM Jiangyong Jia via Star-fcv-l <star-fcv-l AT lists.bnl.gov> wrote:

Hi, Sergei,

1) We have provided an upper limit estimate based on 1/Nch scaling assuming most peripheral bin is all non-flow for v2 (1/Nch scaling is seen in HIJING and also roughly seen in Isobar, though latter show jet quenching effects).  For fixed pT, u and Q, <UQ*>are the same, so it is what is done in both studies.

We just say the effect is only a few percents in the ratio of UU/AuAu, since only the DIFFERENCE matters.

2) Particles after jets quenching is correlated with geometry, i.e. v2 \propto e2 as we know.

3) When jet are quenched and become medium particles. They will also flow and become correlated with participant plane. 

What we mean is experimentally you can not precisely define non-flow in central AA collisions (ALICE for Xe+Xe/Pb+Pb did not do non-flow subtraction).

4) "surface emission" is not flow.  By that we mean correlation between fragments within unmodified jets is non-flow. But of cause the leading particle from surface emission will have a non-zero v2 and is correlated with e2. 

5) hot spots" effect (mentioned in relation to 2-particle CF) is non-flow (again I could have misunderstood it) -- this is incorrect.

We agree correlation arising from hot spot at initial condition is flow.

Jiangyong

On 1/24/24 11:21 PM, Sergei Voloshin via Star-fcv-l wrote:

Hi all,

There was a rather interesting discussion at today's meeting (thanks to Fuqiang and Jianyong for preparing the presentations!), though I had to leave after 2.5 hours...  I have a few comments (not in any particular order) below.

- The discussion could be much more productive, if somebody would chair the session. It could be better if during the presentation only short questions  were allowed, and longer discussions delayed for later. Especially when it is clear that people agree to disagree (but not only).

- I agree with Fuqiang that a clear estimate of nonflow should be made, e.g.for reported v_2, but for other quantities as well.

- For the nonlfow estimate HIJING can be used, though the data might be preferable. From my point of view, using pp data would be the best if used <uQ*>, AA-pp method. One can use peripheral collisions, as tried by Jianyong, including even the scaling presented (though again much better would be <UQ*> approach). Even better would be to compare different estimates.

- The statements were made that it is not really possible to define nonflow. Very strange - what do we report then? Let us define what we want to measure, e.g. the correlation with the participant plane (another possibility would be the correlation with the reaction plane), then what is extra in e.g. measurements with two-particle correlations would be non-flow. 

-  Again, if we report v_2, let us define what we mean by that and report a realistic estimate of "non-flow" in that measurement. The values around a few percent for (ultra)central collisions look very unrealistic, to say the least.

- The paper mentions "standard" and subevent methods, but neither is defined (unless I missed that).

- The effect of nonflow for AuAu and UU collisions is likely rather similar, and the effect on the ratios should be calculated taking this into account. (This is probably what Jianyong meant by "difference" in nonflow)

- Statement that "surface emission" is not flow, is incorrect (unless I misunderstood what was said) at least for the definition of flow I used above,  but it indeed depends on the definition of flow.

- The statement that quenching reduces nonflow is (likely) incorrect, in fact it could increase it. The result of quenching is that instead of one high(er) pt particle we have several low pt. But those still have azimuthal angle strongly correlated with "jet" - then the contribution to nonflow would only increase. 

 

- "hot spots" effect (mentioned in relation to 2-particle CF) is non-flow (again I could have misunderstood it) -- this is incorrect. More exactly it again depends on the definition of flow used. If it is correlations relative to the reaction plane, the statement is correct, if it is relative to the participant plane, the hot spots effect is already included in geometry/flow plane fluctuations.

Best regards,

Sergei

-

--

======================================
 Sergei A. Voloshin    

 Distinguished Professor, 

 Department of Physics and Astronomy,

 Wayne State University,
 666 W. Hancock,  Detroit,  Michigan,  48201
 e-mail: sergei.voloshin AT wayne.edu
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Sooraj Radhakrishnan

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Department of Physics

Kent State University

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Physicist Postdoctoral Affiliate

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Lawrence Berkeley National Lab
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Berkeley, CA 94720

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Email: skradhakrishnan AT lbl.gov

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Sooraj Radhakrishnan

Research Scientist,

Department of Physics

Kent State University

Kent, OH 44243

Physicist Postdoctoral Affiliate

Nuclear Science Division
Lawrence Berkeley National Lab
MS70R0319, One Cyclotron Road

Berkeley, CA 94720

Ph: 510-495-2473

Email: skradhakrishnan AT lbl.gov